Gear pump

Abstract

A gear pump includes: a first pump constructed from a first drive gear rotated by driving of a drive shaft and a first driven gear rotatably arranged coaxially with a rotary shaft; and a second pump constructed from a second drive gear rotatably provided coaxially with the rotary shaft and a second driven gear. The rotary shaft allows the first driven gear and the second drive gear to engage with and disengage from each other in a rotation direction by movement to an axial direction. When the discharge pressure is higher than predetermined pressure, rotation of the first driven gear is not transmitted to the second drive gear and only the first pump operates. When the discharge pressure becomes the predetermined pressure or less, the rotation of the first driven gear is transmitted to the second drive gear and the first and the second pumps operate.

What is claimed is: 
     
       1. A gear pump, comprising:
 a drive shaft; 
 a rotary shaft arranged in parallel to the drive shaft; 
 a first pump constructed from a first drive gear, a first driven gear rotatably arranged coaxially with the rotary shaft and a first pump chamber for receiving the first drive gear and the first driven gear, the first drive gear being rotated by driving of the drive shaft, the first driven gear engaging the first drive gear, the first pump chamber having a first suction port via which oil is suctioned in accordance with rotation of the first drive gear and rotation of the first driven gear and a first discharge port from which the oil is discharged; and 
 a second pump constructed from a second drive gear rotatably provided coaxially with the rotary shaft, a second driven gear rotatably provided coaxially with the drive shaft and a second pump chamber for receiving the second drive gear and the second driven gear, the second driven gear engaging with the second drive gear, the second pump chamber having a second suction port via which oil is suctioned in accordance with rotation of the second drive gear and rotation of the second driven gear and a second discharge port from which the oil is discharged, 
 wherein the rotary shaft is configured such that the first driven gear and the second drive gear engage directly with and disengage from the rotary shaft in a rotation direction by being moved in an axial direction, and 
 wherein the gear pump is configured to selectively operate in a first state or second state, 
 wherein in the first state, the first driven gear and the second drive gear are configured to rotate relatively and the rotation of the first driven gear is not transmitted to the second drive gear, thereby operating only the first pump, and 
 wherein in the second state, the first driven gear and the second drive gear are not configured to rotate relatively and the rotation of the first driven gear is transmitted to the second drive gear, thereby operating both of the first pump and the second pump. 
 
     
     
       2. The gear pump as claimed in  claim 1 , wherein the rotary shaft spline-engages with the second drive gear to rotate together with the second drive gear,
 wherein the gear pump further comprises: 
 an engaging section for causing the rotary shaft to engage with or disengage from the first driven gear with movement of the rotary shaft, 
 wherein, when the rotary shaft is positioned at a first pump side, the engaging section causes the rotary shaft to spline-engage with the first driven gear, whereby the second drive gear and the first driven gear are not allowed to rotate mutually, and 
 wherein, when the rotary shaft is positioned at a second pump side, the engaging section causes spline engagement of the rotary shaft with the first driven gear to be released, whereby the second drive gear and the first driven gear are allowed to rotate mutually. 
 
     
     
       3. The gear pump as claimed in  claim 1 , further comprising:
 two oil chambers for respectively applying hydraulic pressures to both ends of the rotary shaft, 
 wherein the gear pump is configured so that the rotary shaft moves in the axial direction in accordance with loads applied to the both ends of the rotary shaft due to a difference between pressures of the two oil chambers. 
 
     
     
       4. The gear pump as claimed in  claim 3 , wherein an oil passage in which the oil discharged from the first pump flows is connected to each of the two oil chambers, and the gear pump is configured so that the rotary shaft is positioned at the first pump side when discharge pressure of the first pump becomes predetermined pressure or lower. 
     
     
       5. The gear pump as claimed in  claim 1 , comprising:
 a first oil chamber for applying hydraulic pressure to the end surface at the first pump side of the rotary shaft; 
 a second oil chamber for applying hydraulic pressure to the end surface at the second pump side of the rotary shaft; 
 a first oil passage that supplies oil discharged from the first pump to the first oil chamber; 
 a second oil passage that supplies oil discharged from the first pump to the second oil chamber; and 
 a regulator valve provided in the second oil passage, wherein 
 when oil pressure of oil charged from the first pump and supplied to the second oil chamber via the second oil passage become higher than a predetermined pressure, the regulator valve closes the second oil passage, thereby stopping supplying oil pressure to the second oil chamber. 
 
     
     
       6. The gear pump as claimed in  claim 1 , comprising:
 a first oil chamber for applying hydraulic pressure to the end surface at the first pump side of the rotary shaft; 
 a second oil chamber for applying hydraulic pressure to the end surface at the second pump side of the rotary shaft; 
 a first oil passage that supplies oil discharged from the first pump to the first oil chamber; and 
 a second oil passage that supplies oil discharged from the first pump to the second oil chamber, wherein 
 an area of the end surface at the first pump side of the rotary shaft is smaller than an area of the end surface at the second pump side of the rotary shaft, and 
 when a discharge pressure of the first pump becomes predetermined pressure or less, the rotary shaft is positioned at the first pump side.